The invention relates generally to X-ray units and more specifically to shielding for X-ray units.
In an X-ray tube, electrons are produced at a cathode by heating a filament. The electrons are attracted to an anode target by a high voltage potential difference (typically about forty to one hundred fifty kilovolts). When the accelerated electrons hit the anode target, X-rays are produced. Only about one percent of the electron energy is converted into X-ray radiation. The remaining energy is converted into heat. To avoid exceeding the melting point of the focal spot on the target where electrons hit, the target is rotated. The higher the target speed, the lower the focal spot temperature. To avoid scattering the electron beam, the cathode and anode are kept in vacuum conditions (typically about 5xc3x9710xe2x88x927 Torr) within a frame (an X-ray tube).
A xe2x80x9ccracklexe2x80x9d or xe2x80x9cspitxe2x80x9d in an X-ray tube occurs when an arc develops within the tube and effectively short circuits the high voltage power supply. Within about two nanoseconds, the results are that the rotor surface experiences a sudden increase in voltage of about seventy-five kilovolts and that a current of about one thousand amperes flows through the center of the rotor. The pulse remains for about one hundred nanoseconds before being extinguished.
When an X-ray tube xe2x80x9ccrackles,xe2x80x9d a large current pulse flows axially through the tube, couples to the stator, and can cause damage to electronic components in the associated converter.
Commonly assigned Guerin et al., U.S. Pat. No. 5,206,892 describes X-ray tube xe2x80x9ccracklingxe2x80x9d and several conventional approaches to reducing the effects of crackling. These techniques include using metal frames in combination with filters at the inputs of electronic equipment and mechanically fastening/mounting metal screens between the rotor and stator. The shielding discussed in Guerin et al. is electrostatic shielding. Guerin et al. appears to relate to a glass frame assembly (with a joint apparently melted to the stationary stem) having a conventional insulating bell-shaped part between the X-ray tube frame and the stator. Guerin et al. describes coating the bell-shaped part""s outer diameter with a conductive layer to serve as an electrostatic screen and ground-connecting the conductive layer to an X-ray unit housing. In a variant, Guerin et al. describes a metal film being applied to the pre-insulated coil, cable, and magnetic circuit of the stator. In both embodiments, a discontinuity is present for the stated purpose of preventing currents from being induced, in the conductive layer/metal film. The material of the metal film is described as copper, silver, or any other material that is a good conductor of electricity with a thickness range from some micrometers to some tenths of a millimeter.
Erdman et al., U.S. Pat. No. 5,661,353 describes a technique for electrostatically shielding an AC motor wherein capacitive coupling between a stator and a rotor is reduced by positioning an electrically conductive shield therebetween and grounding the shield. Erdman et al. describes the shield as substantially blocking the electric fields produced by charges on the surfaces of the stator windings but allowing the magnetic fields produced by the alternating currents in the stator and rotor windings to pass between the stator and rotor. In one embodiment of Erdman, a plurality of shield members are positioned (and connected) on the internal surfaces of the stator. The shield members are connected to the stator by steel screws or an adhesive and are grounded. In an alternative embodiment of Erdman, a single cylindrical electrostatic shield is used.
It would be desirable to have an improved design for X-ray unit shielding. In Guerin et al., it was not recognized that electrostatic effects are not the only result of crackling, and, because of the discontinuity in the films, shielding against magnetic fields cannot be effectively accomplished. Similarly, Erdman et al. does not describe magnetic shielding. Although an electrostatic screen was a significant improvement over the prior art at the time of Guerin et al., the inventors of the present invention have since discovered that crackling results in electromagnetic as well as electrostatic effects.
Absent shielding, both electric and magnetic fields couple from the X-ray tube to the stator as a result of crackling. In metal frame X-ray tubes (as compared with glass frame X-ray tubes), the metal frame itself will serve as a reasonable electrostatic shield but will not effectively exclude magnetic fields.
The large current pulse that results from crackling creates a circumferential magnetic field pulse which will- not be shielded by a non-magnetic frame. The magnetic field pulse will couple with the stator winding and induce a current pulse in the stator. This current pulse can travel through cabling back to the power electronic converter(s).
In accordance with one embodiment of the present invention, an electric machine comprises a rotor, a stator, and an electromagnetic shield situated between the stator and the rotor to minimize coupling to either one of the stator or the rotor of high frequency magnetic fields resulting from the other of the stator or the rotor with minimal influence on electromagnetic coupling at electric drive frequencies.
In accordance with another embodiment of the present invention, an X-ray tube comprises: a rotor; a rotating anode coupled to the rotor; a metal frame enclosing the rotating anode and the rotor; and an electromagnetic shield comprising an electrically conductive film situated on an outer diameter of the metal frame for shielding magnetic fields resulting from X-ray tube arcing.